1. Field of the Invention
The present invention relates to a digital-signal nonlinear pre-emphasis/de-emphasis apparatus used for eliminating noise components from video signals in a video signal processing system such as a video tape recorder.
2. Description of the Prior Art
Recently, a variety or prior art systems have been proposed for digitally processing video signals while either recording or reproducing video signals in conventional consumerstandard video tape recorders designed for recording analog video signals on a magnetic tape.
More particularly, in a recording mode, an input analog video signal is converted to a digital signal to be subjected to various digital signal processes required for recording, and reconverted to an analog signal before eventually being recorded on a magnetic tape. Conversely, in a reproduction mode, an analog video signal reproduced from a magnetic tape is converted to a digital signal to be subjected to various digital signal processes required for reproduction, and reconverted to an analog video signal before eventually being outputted to the TV monitor. Actually, by digitally processing video signals instead of the conventional analog video signal processing and by introducing large-scale digital ICs, a drastic reduction of the components required for a video tape recorder, and significant improvement of performance and stability of a video tape recorder can be realized. For example, refer to a technical report entitled; "Digital Signal Processing in Video Tape Recorders" written by S. Mehrgart, IEEE Trans. on Consumer Electronics, Vol. CE-31, No. 3, pages 374 through 377, published on August, 1985.
In order to effectively reduce noise from video signals during recording or reproduction normally, conventional consumer-use video tape recorders incorporate non-linear pre-emphasis and de-emphasis circuits.
More specifically, in the recording mode, the non-linear preemphasis circuit emphasizes the high-frequency components of a video signal in such a way that the amount of emphasis becomes larger as the signal amplitude becomes smaller, thereby allowing the needed emphasis without expanding the dynamic range of the video signal. In the reproduction mode, the non-linear de-emphasis circuit de-emphasizes in a way opposite to that of the emphasis process. These processes significantly suppress noise components having small amplitudes and also effectively suppress waveform distortion of signal components having large amplitudes. Since the pre-emphasis and de-emphasis characteristics are opposite to each other, basically, the signal components remain free from degradation.
The non-linear pre-emphasis circuit mentioned above has already been realized by effectively utilizing the non-linear characteristic of diodes. Likewise, a non-linear de-emphasis circuit having characteristics opposite that of the non-linear pre-emphasis circuit has also been realized by inserting a non-linear pre-emphasis circuit into the negative feedback path of an operational amplifier.
However, when digitally processing video signals during the recording and reproduction modes, there arises a problem regarding how to realize a digital non-linear pre- emphasis circuit and a digital non-linear de-emphasis circuit which have opposite characteristics.